The present invention is directed to avionic systems for measuring above ground level (AGL) altitude and ground velocity of an aircraft in general, and more particularly, to combined laser measurement apparatus for measuring both above ground level (AGL) altitude and ground velocity of the aircraft and a fiber optic filter edge detector of Doppler shifts for use therein.
Today's military and commercial aircraft desire more precise measurements of aircraft position and position information. Ground based RADAR systems and Global Positioning Systems (GPS) allow for precise positioning of an aircraft in latitude and longitude desirable for air traffic control, aircraft separation, and navigation. However, precise AGL measurements are often difficult to achieve with such systems, especially for applications in which precise placement above the ground is needed. New levels of precision for altitude or AGL measurements, on the order of +/−6 inches (15 cm), for example, are often required for flight profiles ranging from hover, to nap of the earth (NOE) flight, and autonomous landing. Current aircraft altimeter systems generally can not achieve these precise measurements.
Recently, laser-based altimeters have been proposed for use on-board aircraft. This laser altimeter technology presents a significant advancement over radar altimeters as the ground registered data contains a higher level of resolution due to the narrow beam of the laser. However, the laser altimeters do pose certain concerns when applied to aircraft, especially with regard to the volume of the instrument attributed to the large number of optical elements contained therein. Another concern is directed to the ruggedness of the instrument in an aircraft flight environment. The optical elements of the laser altimeter are generally mounted on an optical bench, adjusted to be precisely aligned with respect to each other and secured in place. But, because of the vibration, shock and wide temperature variation encountered in aircraft flight, the optical elements have a tendency to become misaligned over time and thus, require constant maintenance. In bi-static laser altimeter configurations, back scattering of laser beam transmissions into a telescope portion is an additional concern.
A laser altimeter which overcomes the aforementioned concerns of laser altimeters by providing a compact laser altimeter which improves upon size, ruggedness and maintenance of the instrument is described in the co-pending U.S. patent application Ser. No. 10/386,334, filed Mar. 11, 2003, entitled “Compact Laser Altimeter System” and assigned to the same assignee as the instant application.
Laser systems have also been proposed for use on-board the aircraft in measuring the ground velocity thereof. These ground velocity laser systems propose to use the backscattering of laser emissions off of the ground, similar to laser altimeters, to measure the ground velocity. However, such laser based ground velocity measurement systems usually have more stringent optical alignment concerns than those for the laser altimeters described above.
The present invention overcomes the aforementioned concerns by integrating the capability of measuring ground velocity into a laser altimeter system, such as the system described in the co-pending patent application Ser. No. 10/386,334, for example, to effect a laser based system for measuring both AGL altitude and ground velocity in a common instrument. Through use of fiber optical and signal processing elements, the resulting combined instrument maintains substantially the features of small size, ruggedness and ease of maintenance.